1. Field of the Invention
The present invention relates to an optical switch for switching light paths in optical communication and a method of using the same.
2. Description of the Related Art
There is proposed an optical switch which uses a non-linear optical thin film and utilizes a total reflection phenomenon involved in a change in optical index of the non-linear optical thin film due to excitation light. Japanese Patent Laid-Open Publication No. 2003-228088 discloses an optical switch of a type which obliquely enters signal light to a non-linear optical thin film by using a waveguide and switches a destination of the signal light to a transmitted light side or a reflected light side by irradiating excitation light.
In a case where the optical switch of the type described above is used, the total reflection phenomenon occurs under the following conditions. Specifically, it is assumed that an optical index of an optical material (including a waveguide) in front or back of a thin film material is n1 and an optical index of a thin film is n2. And, when it is assumed that an incidence angle from the optical material (including a waveguide) in front or back of the non-linear optical thin film material to the non-linear optical thin film material is θ1, a refraction angle θ2 in the non-linear optical thin film is expressed by the following equation (1).θ2=a sin−1{(n1/n2)sin θ1}  (1)
To reduce a reflection loss at the time of no excitation, an optical design is made under a condition of n1/n2, so that θ1≈θ2 at the time of no excitation, but the relation becomes θ2>θ1 because the optical index n2 of the non-linear optical thin film lowers at the time of excitation. Here, when the ratio of n2 and n1 is in a region smaller than the condition of the following equation (2), a total reflection phenomenon is caused.(n2/n1)=sin θ1  (2)
For example, total reflection occurs when (n2/n1) is 0.86 or less if θ1=60°, (n2/n1) is 0.974 or less if θ1=77°, or (n2/n1) is 0.999 or less if θ1=88°. In other words, a change in optical index for the total reflection can be made smaller by increasing an incidence angle to the non-linear optical thin film.
Japanese Patent Laid-Open Publications No. 2003-228088 and No. 2004-133329 disclose an optical switch using a non-linear optical thin film that the optical index n2 is changed by 2% or more at a very high speed of 10 nanoseconds. And, the Publication No. 2004-133329 describes that the non-linear optical thin film is formed of fine particles having a particle diameter of 25 nm or less.
In the case of the above-described optical switch, the visible light for excitation is partly absorbed by the non-linear optical thin film and converted to heat. Where the excitation light is intermittently irradiated, it is scattered and cooled down by heat transmission or the like while it is not being irradiated. But, when the above-described optical switch is used under a condition that excitation is continued for a long time, the temperature of the non-linear optical thin film increases gradually, and a change in optical index due to the temperature change overlaps. The temperature change induces a change in polarizability of the non-linear optical thin film, thereby increasing the optical index. A temperature coefficient factor of an optical index of a glass material is expressed by the following equation (3) in a paper in Physical Chemistry of Glasses Vol. 1 (1960) pp 119, or the like.dn/dT={(n2−1)(n2+2)/6n}×{(1/P)·(dP/dT)−3α}  (3)
In the equation, n is an optical index, P is a molar polarizability, and α is a thermal expansion coefficient. In other words, it means that a change in polarizability and a density drop due to thermal expansion give opposite effects on the temperature dependency of the optical index. Here, an oxide material such as Fe2O3, which is used for the above-described optical switch and shows a non-linear optical effect, surpasses in a polarizability effect, and dn/dT has a positive value.
There is also proposed a heat-modulation-type optical switch in that a change in optical index due to heat is used for switching. For example, Japanese Patent Laid-Open Publication No. Hei 9-105891 discloses an optical index modulation element using poly-siloxane of which optical index lowers when heated. The disclosed temperature coefficient factor of the optical index is negatively large to be −1000×10−6 to −50×10−6/° C. But, a change in optical index due to heating and cooling has a slow response speed in order of msec in comparison with a change in optical index caused by light excitation. Switching of this material at a response speed of msec or below due to a change in optical index is difficult.
An object of the invention is to avoid a malfunction of a total reflection type optical switch under excitation conditions for a long time.